The scale on a map is 3cm : 3km. If two cities are 27cm apart on the map, what is the actual distance in km between the two cities?
The scale means that every 3cm on the map corresponds to an actual distance of 3km. A distance of 27cm on the map is the same as 9 $\cdot$ 3cm. The actual distance between the two cities is 9 $\cdot$ 3km, or 27km.